1. Field of the Invention
The present invention relates to a device for redirecting sheets in a sheet transport mechanism, and especially sheets of a print medium in a printing system, such as an inkjet printing system. Further, the present invention relates to a sheet transport mechanism and to a printing system that includes such a device to improve and/or optimize productivity of the system.
2. Description of Background Art
Deformations present within a sheet of a print medium in a printing system can be problematic for various reasons. Firstly, one or more such deformations can cause serious reliability problems in a printing system, such as an inkjet printing system, where there is only a small gap between a sheet transport mechanism and an image forming device or printing head of the printing system. If the sheet to be printed touches the image forming device or the printing head as a result of such a deformation, this can lead to print quality degradation and/or to a sheet jam in the machine. To achieve high print quality in an inkjet printing system, the distance between the printing heads and sheet to be printed should be kept small. Because of this small distance (print gap) the print heads are easily touched by the sheets as they pass. Accordingly, even small defects like dog ears, wrinkles, tears, etc. can cause a so-called “head touch”, which can degrade print quality, cause nozzle failure, or even sheet jams. Secondly, if the sheets of a printed medium output from the printing system include any such deformations, this naturally compromises the quality of the output. Depending on the degree or extent of the deformations in the printed sheets, therefore, those sheets may need to be discarded and re-printed.
To address these issues, systems have been developed, which employ a proofing device capable of identifying sheet deformations and rejecting sheets that contain such deformations. Rejecting one or more sheets, which have been identified as having unacceptable defects or deformations, then involves removing these sheets from a transport path through the printing system. This task may, for example, be performed by a device for redirecting the defective sheets to an alternative path, e.g. to a discharge path. Some conventional redirection devices have been found to exhibit reliability problems, however, in printing applications where the sheets have a relatively high feed rate, e.g. of over 200 sheets per minute. In this regard, conventional devices have been found to experience sheet jams, even when the sheets themselves have not appeared to be sufficiently defective or deformed to cause such a jam, an issue which has confused and confounded designers.
U.S. Pat. No. 6,325,371 B describes a conveying path changing device, having a plurality of branched paths branched from a main conveying path, and a plurality of oscillatable flappers for selecting between a changed position where the sheet is guided from the main conveying path to one of the branched paths and a retracted position permitting passage of the sheet. A single solenoid oscillates the plurality of flappers, while a sensor confirms movements of the flapper by detecting a movement of the link. Before the flappers strike against abutment stoppers, the actuating speeds of the flappers and the links are decreased to suppress the impact noise.